Regarding a Si (silicon) MOSFET (Metal Oxide Semiconductor Field Effect Transistor), which is a power semiconductor device used widely, a main determination factor for breakdown voltage is the upper limit of an electric field strength with which a drift layer serving as a breakdown voltage holding region can withstand. A drift layer made of Si can be broken at a portion fed with an electric field of about 0.3 MV/cm or more. Accordingly, it is required to suppress the electric field strength to be less than a predetermined value in the entire drift layer of the MOSFET. The simplest method is to provide the drift layer with a low impurity concentration. However, this method provides a large on-resistance of the MOSFET, disadvantageously. In other words, there is a trade-off relation between the on-resistance and the breakdown voltage.
Regarding a typical Si MOSFET, Japanese Patent Laying-Open No. 9-191109 illustrates a trade-off relation between the on-resistance and the breakdown voltage in consideration of a theoretical limit resulting from a property value of Si. In order to cancel this trade off, it is disclosed to add a lower p type embedded layer and an upper p type embedded layer in an n type base layer provided on an n type substrate provided on a drain electrode. By the lower p type embedded layer and the upper embedded layer, the n type base layer is divided into a lower stage, a middle stage, and an upper stage, each of which has an equal thickness. According to this publication, voltage is equally held by each of the three stages, whereby the maximum electric field of each stage is maintained to be equal to or less than the critical electric field strength.
Moreover, the publication described above discloses to provide a termination structure having a guard ring (also referred to as “Field Limiting Ring”). Specifically, in the termination structure, guard rings are provided at depth positions respectively corresponding to the three stages described above. More specifically, in the termination portion, embedded guard rings are respectively provided in the n type base layer at two depth positions different from each other, and a guard ring is provided also at a surface of the n type base layer. With these three types of guard rings, the maximum electric field of each stage is also maintained to be equal to or less than the critical strength in the termination structure.
In addition, more generally, a termination structure having a guard ring only at a surface of an n type base layer without having the embedded guard rings described above has been used more widely.